As the system of directly plate-making from digital signals such as computer data, systems by (1) electrophotography, (2) photopolymerization by combination of exposure by Ar laser and post-heating, (3) lamination of a silver salt photosensitive material on a photosensitive resin, (4) a silver master type, and (5) breakdown of a silicone rubber layer by discharge breakdown and laser beams are known.
However, a method of using electrophotography of (1) is complicated in processes such as electrification, exposure and development, and requires intricate and large-scaled equipment. A method (2) requires highly sensitive printing materials over and above a post-heating process, and handling in daylight is difficult. Since methods (3) and (4) use silver salts, processing is complicated and uneconomical. A method (5) is a method of comparatively reaching completion, but a problem of the elimination of silicone residues on a plate remains.
On the other hand, the development of laser in recent years is remarkable, in particular, small-sized solid state lasers and semiconductor lasers having emission regions in near infrared to infrared of high output are easily available. The laser is very useful as the exposure light source of direct plate-making from digital signals such as computer data in the aspects of the compaction of the plate-making system, the environmental lights at plate-making work and the costs of plate-making materials.
As conventional lithographic printing plate materials, Japanese Patent Publication No. 27919/1971 discloses a method of forming an image by heating, in accordance with data, a printing material containing a recording layer to which a polymer compound or a composition which is insoluble or slightly soluble before heating and becomes more soluble in a solvent under heating is mixed, and Japanese Patent Laid-Open No. 69192/1981 discloses a heat-sensitive recording material having a heat-sensitive layer containing a novolak type phenol resin and a carbon black. However, these patents disclose only the examples of the case where images are recorded without using laser beams, and when images are recorded by lasers having emission regions from near infrared to infrared as the exposure light sources in the case of directly plate-making from digital data such as computers, good printed substances cannot necessarily be obtained due to background staining and the reduction of press life. For obtaining good printed substances, it is necessary that an exposed area (a non-image area) should be easily dissolved and an unexposed area (an image area) remain by alkali development processing after exposure and the remaining image area have good durability. That is, it is thought that since the recording property of the image is not good, the non-image area is dissolved with difficulty and the image area is easily dissolved when laser beams are used in the above well-known techniques.
To cope with these problems in prior techniques, Japanese Patent Laid-Open No. 285275/1995 discloses to use, as the image-forming layer, an image-recording material containing a binder, a substance which absorbs light and generates heat, and a substance which is heat-decomposable and substantially reduces the solubility of the binder in the state not being decomposed, to thereby obtain a lithographic printing plate material having good recording property capable of directly plate-making from digital signals such as computer data by using conventional processing apparatus and printing press as they are.
Further, Japanese Patent Laid-Open No. 215693/2001 discloses an image-forming material having a positive image-forming layer containing an acid generator and an acid-decomposable compound.
Further, Japanese Patent Laid-Open No. 282643/1998 discloses that for a positive photosensitive composition containing a light-to-heat converting substance and an alkali-soluble resin to contain an organic acid, the residual film rate in an unexposed area can be improved.
Moreover, Japanese Patent Laid-Open No. 324808/2001 discloses a positive photosensitive composition containing in specific weight ratios of a light-to-heat converting substance, a novolak resin, and an acrylate resin containing a (meth)acrylic acid and a (meth)acrylic ester, and high speed and the improvement of the residual film rate in an unexposed area can be obtained.
However, in the above-disclosed techniques, further improvement is desired in the points of processability (development latitude) and press life in the case where the activity of a developing solution changes.